Mercury is an impurity at low concentration in the earth's crust. Mercury is present in three basic forms: metallic; inorganic mercury in Hg+1 or Hg+2 valence states (e.g., as an inorganic chloride); and organic mercury bound to phenyl-, alkoxyalkll-, or methyl-groups. Methyl mercury and elemental mercury are the most hazardous forms.
Major sources of mercury pollution include impurities in or contamination of industrial processes, such as gaseous effluent from burned coal and from chlor-alkali plants that can become entrained in liquid process streams such as from wet scrubbers, as well as liquid effluent from industrial processes, such as mining operations and crude oil drilling. Another source is accidental release.
Coal forms by the combination of long-term putrefaction and pressurization under reducing conditions of prehistoric buried organic plant matter. Given the nature of the natural process that makes coal and the high solubility of mercury in organic solvents, mercury often finds its way into coal. The solubility of mercury in benzene, heptane, isopropyl ether, and iso-octane is between approximately 1 to 2.5 mg/l. Its solubility in water is approximately 0.064 mg/l. While mercury exists in very small concentrations in coal, the massive volume of coal burned for power generation yields a significant (i.e., 40% or greater) overall emission of mercury into the environment.
The two prevalent classifications of coal are bituminous and brown (i.e., lignite or sub-bituminous). Bituminous coal from the eastern U.S. contains primarily ionic mercury. Sub-bituminous coal, mainly from the western U.S., yields predominately elemental mercury. Sub-bituminous coal, which contains mercury in a more hazardous form, is the predominant source of coal.
Because of the two types of coals and the characteristics of specific power plants, the boiler in a typical power plant releases mercury in both forms, ionic and elemental. Downstream wet scrubbers more readily remove the ionic form, thereby creating a liquid process stream containing mercury. The elemental form of mercury is more difficult to remove from the gas stream. Most methods to remove mercury in the gas phase aim to convert all the mercury to an ionic form. Unless the effluent of the wet scrubber contains substances that bind the ionic mercury (e.g., sulfate anions), the effluent water will be contaminated with the mercury removed from the gaseous state.
Mercury may also be released into the environment in soluble forms when it has been oxidized and/or converted to a soluble salt, such as a chloride, or organic forms such as methyl mercury. These forms may be soluble in organic or aqueous liquids.
The Electric Power Research Institute (EPRI) has examined a number of approaches to mercury removal from flue gas. The steps in the power plant generation cycle involve feeding coal to the combustor, combustion of coal, collection of flue gas, removal of NOX and particulates, removal of SOX, and exhaust to the environment. The complicating factor in this cycle is that coal-fired power plants are of varying age, and some have only part of the pollution abatement methods described below (or in some cases, none at all), depending on age and location. The pollution abatement methods described below address removal of the contaminant from the waste stream from the combustion of coal. The waste stream comprises NOX, SOX, coarse ash, fine fly ash, CO2 and mercury.
An important consideration is how removal of mercury impacts the quality of fly ash and gypsum (calcium sulfate from SOX removal). Primary markets for fly ash and gypsum are as a substitute for cement in concrete, and from gypsum as wallboard and soil amendments. If mercury is bound to fly ash or enters the SOX scrubbers it may ruin the use of these components in these applications.
Known methods to remove mercury from waste streams are as follows:
Coal Cleaning. Bituminous coal is cleaned routinely prior to combustion to remove non-combustibles. Although not intended for the purpose, this cleaning removes up to approximately 35% of the mercury. EPRI states it is unlikely to achieve a higher reduction in mercury in bituminous coal by cleaning. In contrast, sub-bituminous coal is usually not cleaned. De-watering processes under development for sub-bituminous coal may have the potential to remove up to approximately 70% of the mercury.
Additives To Oxidize Mercury. An oxidizer (e.g., salts, such as chloride) may be added to oxidize the mercury and convert it to ionic form. This makes the mercury more susceptible to removal by scrubbers and other methods described herein, which remove mercury in ionic form.
Modify the Combustion Process. Activated carbon is effective to remove mercury. Increasing the content of un-oxidized carbon in the flue gas by modifying the combustion process enhances removal of the mercury in this manner. However, the mercury-laden particulate in the fly ash renders the fly ash unusable. Changing the oxidation/reduction character of the combustion process also leads to lower efficiency.
Selective Catalytic Reduction (SCR). Another approach oxidizes mercury in the SCR (which converts NOX). Down-stream wet scrubbers collect the oxidized mercury in an aqueous stream. An alternate approach uses a mercury-selective catalyst in the gas stream for this purpose. Typically this involves a “fixed absorbent structure” with plates or channels lined with the adsorbents such as gold, sulfur or activated carbon. A major issue with SCR for oxidation for mercury is whether such devices can maintain selective oxidative power over a reasonable life, i.e., approximately 12,000-16,000 hours (12-22 months), and whether sufficient contacting of adsorbent with mercury can be achieved.
Sorbent Injection. Activated carbon is a very good adsorbent of mercury. However, the cost of activated carbon is a significant issue. An EPRI publication cites short-term tests that removed up to 80-85% of mercury from bituminous coal fired plant operations by injecting activated carbon as a fine powder in the flue gas. However, the removal of mercury in western coals peaks at 65-70%. This method requires injection of a sizable quantity of expensive carbon “dust.” A further complication of using this method, or any method that injects activated carbon upstream, is that the carbon with adsorbed mercury contaminates the collected ash in the latter stages of the flue gas cleaning process, rendering the fly ash commercially useless for the largest current application (i.e., as a substitute for cement in concrete). This method thus may require an additional step of removal of the mercury from the ash, such as using sulfur-added (or bromine-added) activated carbon. The efficiency of this additional step is debatable. The durability of the injection process also is not well known and is an area of active development. The necessity to control location of the activated carbon injection into the waste steam to avoid contaminating the fly ash with mercury is a disadvantage. The carbon might be injected after the electrostatic precipitator (ESP) to avoid contaminating the fly ash, but this still requires a “polishing” fabric filter to remove the carbon holding the captured mercury. The filters, however, may increase back pressure of the flue. While some polishing filters being tested report 85-95% efficiency in short term tests, full scale, long term tests have not been completed.
Electrostatic Precipitators. The ESP is virtually useless for removing mercury unless some upstream process is used to bind mercury to particulates, such as, for example, activated carbon injection. Typical efficiency for mercury removal is from 0% to approximately 35% for ESP without particulate binding. The efficiency of the process using fabric filters increases removal to approximately 35-99% for bituminous coal and approximately 48-86% for sub-bituminous coal. When sorbents are used, ESP with fabric filters leads to mercury in the fly ash. As mentioned previously, this contaminates the fly ash.
FGD (Flue Gas Desulphurization) Additives and Scrubbers. This developing technology injects active material into the liquid in the SOX scrubbers, which remove SOX, primarily as sulfate. The additive reacts with the mercury to form non-volatile salts. The reaction must be fast enough to avoid contaminating the calcium sulfate that forms in reaction with the slurried limestone, and thus prevent contamination of the resultant gypsum. FGD will remove approximately 90-95% of ionic mercury, but little or no elemental mercury.
Fixed Absorption Structure. In this developing technology, plates or honeycomb structures with mercury-adsorbent materials, such as gold or activated carbon, are placed in the flue gas stream.
These prior art methods are not completely satisfactory for removing mercury because conventional adsorbents, such as activated carbon, sulfur and elemental gold, each have particular problems, including but not limited to expense, contaminating the fly ash, and related performance issues even when they demonstrate high efficiency at removing mercury from the gas stream. The main reason appears to be that the specific adsorbents work only, or best, with mercury in its oxidized state and do not work very well in its unoxidized or elemental vapor state. Another undesirable characteristic of activated carbon is that mercury is typically physically adsorbed (physisorbed) to it, not chemically adsorbed (chemisorbed). This means the mercury is not strongly bound and may be removed by physical actions such as washing or contacting the activated carbon with a mildly reactive chemical, thus making the activated carbon a potential hazard.
Another industrial source of mercury contamination are chlor-alkali plants that use liquid mercury in an electrochemical process to produce sodium hydroxide and chlorine. These have the potential of mercury in process streams. Mercury concentration in the air on roads adjacent to two chlor-alkali plants has been reported at 1,788 ng/m3 and 2,629 ng/m3, both being far above the EPA reference concentration for chronic mercury exposure of 300 ng/m3 and the Agency for Toxic Substances and Disease Registry (ATSDR) safe level for chronic exposure of 200 ng/m3. The EPA states that the most significant potential emission point in chlor-alakli plants is thought to be the Hg cell building roof vent. This implies a primary source is gaseous mercury. Although these reports suggest most of the mercury is emitted in gaseous form, it may become part of a liquid stream in the plant, or in the run-off of water in the general area adjacent to the plant where high air concentrations of mercury are found.
Sulfide precipitation appears to be the common practice for mercury control in many chlor-alkali plants, and achieves levels of 95-99.9% reduction for well-designed and managed treatment. Such methods typically use sodium hydrosulfide or magnesium sulfide to form a relatively insoluble mercury sulfide, HgS, which precipitates and forms a sludge. Studies have cited examples where initial levels of 10 ppm (10 mg/L) are reduced to 10-100 ppb (10-100 μg/L). However for effect, these processes must work at a pH less than 9. This type of treatment creates significant mercury-laden sludge that in itself is a potential environmental hazard if placed in landfill because it may create mercury leachate and ground-water pollution. In addition, this sulfide precipitation method appears not to be able to reduce mercury below 10-100 ppb (10-100 μg/L).
In a separate report from Oak Ridge National Laboratory, mercury concentration in wastewater varied between 105 to 837 ng/liter (parts per trillion, ppt) while the EPA requirement is no more than 19 ppt. Thus, industrial operations may yield mercury pollution in both aqueous and gaseous state. The report described a variety of other remediation methods, including other precipitation methods, although these were not substantial improvements over sulfide precipitation. Included in these other remediation methods were adsorption processes using activated carbon in either granular form (GAC) or powder form (PAC). These methods used filter beds and a micro-filtration process to capture lost activated carbon containing mercury. The best method using a 10 ppm (10 mg/L) input achieved an output mercury concentration of approximately 0.2-1.0 ppb (0.2-1.0 μg/L). To achieve such low levels, PAC is soaked in CS2 and filtered. The performance is attributed to chemisorbed mercury to the CS2. The study suggested that adsorption using activated carbon drops as the solution pH deviates much from 4-5. A major drawback of this approach is that the activated carbon cannot be regenerated economically. In addition, the carbon works by the principle of physisorbing the mercury or the CS2. This means that the carbon has the same problem as sludge from precipitation. If it is disposed the potential for leaching and ground-water pollution cannot be ignored.
While ion exchange methods, at least on bench scale, can achieve 0.4-1 ppb (0.4-1 μg/L) final concentrations, they cannot be used in aqueous streams with high solids content, create mercury-contaminated brine when regenerated, and can exhibit substantial variability. These methods rely on the exchange of the mercury cation in a soluble form, and thus they work mostly in high chlorine streams.
A variation of the precipitation method uses small magnetic particles to act as nucleation sites for a coagulation or precipitation reaction involving the mercury. These mercury-laden precipitates are filtered from the stream. For an input concentration of 15 mg/L, this method on the bench scale reported a final concentration of mercury of 3 ppB (μg/L) to 0.117 ppm (mg/L) when used on a waste stream of a municipal solid waste incinerator. These precipitates have similar problems as previous methods: they pose disposal problems and do not demonstrate very low final mercury levels.
Accordingly, what is needed is a method of removal of a contaminant such as mercury from process streams that avoids the shortcomings of the known methods described above. In particular, an adsorbent method is needed that allows for more and longer contacts of the adsorbent surface with the contaminant, and that strongly chemisorbs the contaminant so it does not leach or readily regenerate the mercury. It is to such that the present invention is directed.